The present invention relates to compensation for variation in attenuation measurements by a sensor caused by changes in the ambient temperature of the sensor.
In many sensor applications, it is necessary to determine the attenuation of a signal passing from a transmitter to a receiver. One such application is diesel particulate filter (DPF) sensors using radio frequency (“RF”) signals across the DPF. A DPF is a device designed to trap and remove diesel particulate matter (i.e., soot) from the exhaust gas of diesel engines as the exhaust gas passes through the DPF. DPF can be used to reduce emissions and improve efficiency of internal combustion engines and after treatment systems. Since a DPF must periodically be cleaned when the soot loading of the DPF exceeds a certain threshold, DPF sensors can be employed to monitor the soot loading of a DPF. Different types of DPF sensors exist using different technologies to monitor the soot loading of a DPF, including RF technology. The DPF sensor can provide data relating to the amount of soot loading of the DPF to an engine control module (ECM), which can then determine when the DPF should be cleaned.
In a DPF sensor using RF signals, the power of an RF signal transmitted by an antenna located on one side of the DPF is compared to the power of that RF signal received by an antenna located on the other side of the DPF to measure the attenuation in the signal caused by the DPF. The DPF sensor or ECM can then correlate the attenuation caused by the DPF with the amount of soot loading of the DPF. For example, a particular attenuation value caused by the DPF coupled with other data (e.g., temperature across the DPF) indicates a particular amount of soot loading of the DPF.
A transfer function relating attenuation to soot loading for a particular type of DPF can be empirically determined over a range of frequencies at a particular exhaust gas temperature. For example, if an RF signal DPF sensor monitoring a DPF having inlet and outlet exhaust gas temperatures of 250° C. measures an attenuation value of 10 dB (i.e., 10 dB is lost across the DPF) for a 700 MHz signal, and an actual soot load (1.0 g/l) of the DPF is measured under those conditions (e.g., by weighing the DPF), the combination of the particular exhaust gas temperature (250° C.), attenuation value (10 dB), and frequency (700 MHz) is correlated to that measured soot load (1.0 g/l). Keeping the exhaust gas temperature constant, that process can be repeated over a range of soot loads and over a range of frequencies to correlate each attenuation value to a measured soot load at that particular temperature. Then, this same process can be repeated (i.e., measuring the actual soot load of the DPF at different soot loads at different frequencies) at several different exhaust gas temperatures.
Typically, this correlation process for an RF signal DPF sensor would take place in an environment where the ambient temperature of the sensor and its associated electronics is approximately room temperature (25° C.). However, since in the field, the RF signal DPF sensor can be exposed to a wide range of ambient temperatures (e.g., −40° C. to 85° C.), its performance (e.g., sensitivity of detectors, output power of the transmitter) will vary at these different ambient temperatures. For example, while at an ambient temperature of 25° C., an RF signal DPF sensor monitoring a DPF having an exhaust gas temperature of 250° C. measures an attenuation value of 10 dB for a 700 MHz signal for a soot load of 1.0 g/l, under the same exact filter conditions (exhaust gas temperature, filter soot load, and RF signal frequency) but with the sensor at a different ambient temperature, the sensor will measure a different attenuation value, which would then result in an incorrect correlation to soot load based on the correlation process performed at an ambient temperature of 25° C.
One solution to this problem is to perform the same correlation process that was performed at an ambient temperature of 25° C. at various temperatures over the wide range of possible ambient temperatures. However, such an effort would result in significant expenditures of time, resources, and money for each sensor that required correlation. Accordingly, it is desirable to be able to compensate for variation in attenuation measurements resulting from variations in the sensor performing the attenuation measurements caused by changes in ambient temperature without the need for additional measurements at temperatures other than 25° C.